Replicated tests were conducted for three years at the Edisto Research & Education Center of Clemson University to determine the effects of subsurface drip irrigation on soil compaction and optimum depth of drip irrigation tube placement in coastal plain soils. Three lateral depths (8, 12, and 16 inches), three lateral spacings (under every row, under alternate row middles with and without under the row subsoiling) and a control (non-irrigated) were used.

The rate of water movement in the soil profile for plots with deep tillage was higher than plots without tillage. Soil surface moisture was higher in plots with laterals buried eight inches deep resulting in higher weed infestation than the rest of the treatments. Tillage significantly reduced soil compaction in the top 15 inches of the irrigated plot compared to no subsoiling. Also there was a significant reduction in cone index values in the top 12 inches of dry land plots compared to irrigated lands without tillage. Subsoiled plots with irrigation laterals buried 16 inches deep had the least cone index values at depth of 12-18 inches. Deep tillage significantly increased taproot length in irrigated plots. Taproots in no-till plots were restricted to the depth of the hardpan layer. Keeping this compacted layer wet did not reduced soil strength enough to permit root penetrations into clay.

Drip irrigation significantly increased lint yields compared to non-irrigated plots in all three years. Depth of the irrigation tubes had an effect on cotton yield, increasing with depth in both under every row and using alternate row middles. There were no differences in yield between every vs. alternate row installation at any of the three placement depths. Deep tillage did not increase the cotton yield in 1997 compare to no-till planting because all plots in 1997 had some tillage provided during installation of the irrigation laterals. Although not statistically significant, plots with a deep tillage operation on average yielded 47-lb. and 45-lb. lint/acre more than no-till plots in 1998 and 99 respectively.